1. Field of the Invention
The present invention relates to a toner, a developer and an image forming method used for developing an electrostatic latent image in electrophotography and electrostatic recording method.
2. Description of the Related Art
In electrophotography, an image is obtained by developing an electrostatic latent image formed on a latent image holding member (photosensitive member) into a toner image with a toner composition containing a colorant (hereinafter, simply referred to as “toner” in some cases), transferring the resulting toner image onto a transferring member, and fixing this with a heat roll and, on the other hand, the latent image holding member is cleaned for forming an electrostatic latent image again. A dry developer used for such the electrophotography is roughly classified into a one-component developer using alone a toner in which a colorant and the like are incorporated into a binding resin, and a two-component developer in which a carrier is mixed into the toner.
Since late in 1980's, in the market of electrophotography, miniaturization and high performance were strongly demanded as keywords of digitalization and, in particular, regarding full color image, high image quality close to that of high-grade printing and silver salt photography has been desired.
As means for attaining high image quality, digitalization is indispensable, and the performance of digitalization regarding such the image quality includes the ability to perform complicated image processing at a high speed. Thereby, it becomes possible to control a letter image and a photography image separately, and the reproductivity of quality of both images has been greatly improved as compared with the analog technique. In particular, regarding photographic images, the enabling of gradation correction and color correction has been important, and digitalization is advantageous as compared with analog in terms of gradation properties, fineness, visibility, color reproduction and graininess. However, on the other hand, as image output, it is necessary to faithfully make an image from a latent image produced by an optical system, a particle diameter of a toner grows increasingly smaller, and the activities aiming at faithful reproduction are being accelerated. However, mere reduction in a toner diameter hardly affords high image quality stably, and improvement in the fundamental properties in development, transference and fixing properties have become more important.
In particular, in a color image, three-color or four-color toners are overlapped to form an image. For this reason, when any of toners exhibits different properties from original ones or different performance from that of other colors in a viewpoint of development, transference and fixing, reduction in color reproduction, deterioration of the graininess, and deterioration of image quality such a irregular color are caused. In order to maintain a stable high quality image in its original state even over time, how the property of each toner is stably controlled is important.
For example, in the prior art, a toner having an average particle diameter of 7 to 14 μm was proposed (see Japanese Patent Application Laid-Open (JP-A) No. 62-103675). In addition, regarding a particle size distribution, JP-A No. 2-132459 proposed a toner having a sharp particle size distribution.
Generally, as a particle size becomes smaller, the electrifying property of a developer tends to decline as more images are printed, and a life of the developer is shortened. This is because when a particle size becomes smaller, a surface area of a toner per unit becomes larger and an amount of an additive added to the surface thereof becomes also larger, but these allow toner components and additives to easily stain the carrier surface. Therefore, a sharper particle size distribution of a toner is better, but there is a limit due to a problem of manufacturing and the cost. The toner described in the above-mentioned Japanese Patent Applications Laid-open also included a toner whose particle size distribution was broad in the large particle diameter range or in the small particle diameter range. It was thus difficult to obtain a high quality image.
Further, there was a problem that since a fine powder having a small particle diameter was contained, a developer's life was short (for example, see JP-A No. 2-132459).
For the purpose of improving a developer's life, a metal oxide with a chain-like polymer grafted (for example, see JP-A No. 64-9467), and an electrically conductive fine particle covered with a resin (for example, see JP-A No. 4-335649) were proposed. However, uniformity of surface treatment was low, and although a developer's life was maintained to a certain extent, the improvement was not sufficient.
In addition, in order to satisfy both of improvement in the flowability and the environmental stability of electrification, for example, JP-A No. 60-136775 tried to use hydrophobic titania and hydrophobic silica jointly. However, when a color toner having a small diameter was used, mere mixing of them resulted in a worse transferring property, and a stable image could not be obtained.
In addition, in order to satisfy both of improvement in the flowability and the environmental stability of electrification, for example, JP-A No. 10-186723 tried to use a hydrophobic titanium series compound and hydrophobic silica having different particle diameters jointly. However, when a color toner having a small particle diameter was used, mere mixing of them maintained a life of a developer to a certain extent, but the improvement was not sufficient, and a stable image could not be obtained for a long period of time.
On the other hand, JP-A No. 10-312089 reports that a toner is stirred in a developing unit, the fine structure of the toner surface is easily changed, and the transferring property is greatly changed.
Recently, for the purpose of miniaturizing an apparatus from a viewpoint of space saving, decreasing toner wastes from a viewpoint of environmental protection, and prolonging a life of a latent image holding member, a cleanerless system is proposed. In the cleanerless system a cleaning system is omitted, a toner remaining on the surface of photosensitive drum after transference is dispersed with a brush touching the surface of the photosensitive drum, the dispersed toner is recovered in a developing unit at the same time of development (for example, see JP-A No. 5-94113).
Generally, when a remaining toner is recovered at the same time of development like this, the recovered toner and other toner have different electrifying properties, such a problem may occur that the recovered toner is not developed and is accumulated in a developing unit. Therefore, it becomes necessary to further increase the transfer efficiency and to minimize the amount of a toner to be recovered.
In addition, in order to improve the flowability, the electrifying property and the transferring property, for example, JP-A No. 62-184469 proposed to render a shape of a toner approach a spherical shape.
However, rendering a toner a spherical shape may easily cause the following problems. A conveyance amount controlling plate is provided on a developing unit for controlling an amount of a developer to be conveyed constant, and the gap between a developer holding member and a conveyance amount controlling plate is changed so as to control the conveyed amount of a developer. However, when a spherical toner is used, the flowability of a developer is increased and at the same time, a hardening bulk density is increased. As a result, there occurs the phenomenon that, a developer is accumulated at a conveyance regulating site, and a conveyance amount becomes unstable. By controlling the surface roughness on a developer holding member and, at the same time, narrowing the gap between a controlling plate and a developer holding member, a conveyance amount may be improved. However, the packing property due to accumulation of a developer is strengthened increasingly and, accordingly, a stress applied to a toner becomes stronger. It is confirmed that there is such a problem in that the above phenomena make a fine structure change of the toner surface, particularly embedding or peeling, of an external additive occur easily, and that the developing property and the transferring property are greatly changed as compared with an initial stage.
In order to solve these problems, JP-A No. 6-308759 reports that a spherical toner and a non-spherical toner are combined to suppress the packing property and high image quality can be attained. However, although this is effective in suppressing the packing property, a non-spherical toner easily remains as the transference residue, and the high transference efficacy can not be attained. In addition, when development and recovery are performed at the same time, since a non-spherical toner which is the transference residue is recovered, a ratio of a non-spherical toner is increased, causing a problem of further decline in the transference efficacy.
In addition, in order to improve the developing property, the transference property and the cleaning property of spherical toner, JP-A No. 3-100661 discloses that two kinds of inorganic fine particles having different particle diameters of a particle having a volume average primary particle diameter of 5 μm or more and less than 20 μm and a particle having a volume average primary particle diameter of n 20 μm or more and 40 μm or less are used jointly, and they are added at specified amounts. Although this can afford the high developing property, transferring and cleaning property at an initial stage, since a force applied to a toner can not be reduced over time in any cases, embedding or peeling of an external additive easily occurs, greatly changing the developing property and the transferring property as compared with those in the initial stage.
On the other hand, against such a stress, it is disclosed that the use of an inorganic fine particle of a large particle diameter is effective for suppressing embedding of an external additive into a toner (for example, see JP-A Nos. 7-28276, 9-319134 and 10-312089). However, since an inorganic fine particle has a large specific gravity, when a size of an external additive particle is increased, by the stirring stress in a developing unit peeling of an external additive or the like becomes inevitable. In addition, since an inorganic fine particle does not exhibit a completely spherical shape, when attached to the toner surface, it is difficult to control standing of an external agent constant. By this, scattering occurs in a microscopic surface convex shape which functions as a spacer, and a stress is selectively applied to a convex part and, therefore, embedding or peeling of an external additive is further accelerated. This method is not sufficient either.
In addition, JP-A No. 6-266152 discloses the technique of adding an organic fine particle of 50 to 200 nm to a toner in order to effectively manifest the spacer function. By using a spherical organic fine particle, it is possible to manifest the spacer function effective at an initial stage. Embedding or peeling of an organic fine particle hardly occurs against a stress over time. However, since an organic fine particle itself is deformed, it is difficult to stably manifest the high spacer function. There is an idea to obtain the spacer effect by attaching a large amount of organic fine particles on the toner surface or using organic fine particles having a large particle diameter. However, according to this idea, the property of an organic fine particle is greatly reflected. That is, there arises the influence on the powder property such as inhibition of the flowability of a toner with an inorganic fine particle added thereto and deterioration of thermal flocculation. An organic fine particle itself also has the electrification imparting ability and this causes an influence on electrification and development such as a decline in a freedom degree of control from a viewpoint of electrification.
Recently, colorization, particularly on demand printing is highly required. And, for responding to high speed copying, JP-A No. 8-115007 reports a method of forming a multi-colored image on a transferring belt, and transferring and fixing the multi-colored image on an image fixing member at once. Transference is repeated two times: the primary transference comprising of a step of transferring from a photosensitive member onto a transferring belt and secondary transference comprising of a step of transferring from a transferring belt onto a transferring member. The technique of improving the transferring efficacy thus becomes increasingly important. In the secondary transference in particular it is imperative to control the electrification, development and transferring properties in order to reduce the influence thereof, since a multi-colored image is transferred at once, and the properties of recording member (e.g. thickness and the surface property in the case of a paper) are variously changed.
In addition, in order to reduce consumed power and space and obtain a high quality image, the technique of transferring each color onto an intermediate transferring member and fixing it at the same time with transference onto a transferring member is disclosed (for example, JP-A Nos. 10-213977 and 8-44220). Herein, the importance lies in the necessity that a transferring belt has both of the transferring function and fixing function. That is, since it is necessary to improve the transferring property in the cooled state at a primary transference part and transmit the heat instantaneously at a secondary transference and fixing part, as a belt, a thin belt having the high thermal resistance is used. Here, since the transferring efficacy can not be controlled extremely high and a high pressure can not be applied at fixing, the function of adaptation is required for a belt at low pressure fixing. In addition, since the belt surface also has the transferring function, it is important to minimize toner contamination at fixing and scratch due to an external additive.
On the other hand, there is proposed a method of faithfully reproducing high image quality, in particular half tone, solid black and letters by controlling volume specific resistance of a carrier (for example, see JP-A Nos. 56-125751, 62-267766 and Japanese Patent Application Publication (JP-B) No. 7-120086). In any of these methods, resistance is adjusted by a kind and a covering amount of a carrier-covering layer. Although aimed volume specific resistance is obtained and high image quality is manifested at an initial stage, peeling of a carrier-covering layer or the like occurs by a stress in a developing unit, and volume specific resistance is greatly changed. Therefore, it is difficult to manifest high image quality over a long time.
On the other hand, a method of adjusting volume specific resistance by adding carbon black to a carrier-covering layer is proposed by JP-A No. 4-40471.
Although change in volume specific resistance by peeling of a covering layer is suppressed by this method, an external additive added to a toner or toner components attach to a carrier, which changes volume specific resistance of a carrier, and, therefore it is difficult to manifest high image quality over a long time as in the aforementioned carrier.
By the way, since, generally, as a particle size becomes smaller, the flowability is deteriorated, or a surface area of a toner per unit weight increases, an amount of a fine particle called external additive to be added to the toner surface increases. Usually, as this fine particle, an inorganic fine powder or an organic fine particle having high Tg or the cross-linking property is used in order to improve the aforementioned flowability, the electrifying property and the transferring property. The present inventors intensively studied and, as a result, found that when an amount of an external additive to be added is increased by every miniaturization in a diameter of a toner, fixing property is deteriorated.
As a method of fixing a toner image, there is a heating fixing method using a heating roller or a heating film. Currently, a heating roller method that has better thermal efficiency and that high speed fixing ability is widely used. However, a heating fixing method using a heating film (belt fixing, film fixing) has been being adopted due to its merits such as a short waiting time from switch on of electric source to ready to use and a small heat capacity that enables reduction in power consumption. In addition, in fixing of a color fixing, since the surface of a heating fixing member and a toner image in the molten state are contacted under pressure, and a part of the toner image is attached to the surface of a heating member, whereby, an attached toner is re-transferred to prevent contamination of a copied image, so-called offset phenomenon, a method of supplying a releasing oil (releasing liquid such as silicone oil etc.) is adopted. However, although this method is extremely effective to prevent the offset phenomenon of a toner, this releasing oil (offset preventing liquid) produces unpleasant odor when heated and vaporized, and an apparatus for supplying an offset preventing liquid becomes necessary. Therefore, a fixing apparatus without the use of releasing oil is currently being adopted by re-design of a toner resin and a wax resin.
For the purpose of improving the fixing property, a molecular weight of a resin constituting a toner bulk, Tg, and a kind and an amount of a wax are variously studied. For solving such problems, there are proposed a method of limiting the viscosity of a toner (see JP-A Nos. 1-133065, 2-161466, 2-100059 and 3-229265), a method of inclusion of a wax such as a resin having the releasing property (see JP-B No. 52-3304), a method of limiting the melt viscosity of a wax (see JP-A Nos. 3-260659 and 3-122660), a method of limiting a diameter of wax domain and a rate of a wax existing on the toner surface (see JP-A No. 7-84398), a method of limiting a shape of a wax domain (see JP-A No. 6-161145), and the like.
In addition, regarding a heating fixing method using a heating film, there are a variety of proposals from a viewpoint of the more stable fixing property and energy saving. For example, for the purpose of suppressing the offset phenomenon of fixing, JP-A No. 3-122661 proposes a method of limiting the viscosity of a binder resin and a releasing agent constituting a toner.
However, it can not be said that the above method has sufficiently realized the fixing property substantially without using releasing oil. There still remain problems to be solved. Accordingly, a study of toner bulk material has a limit to some extent, and it is difficult to respond to deterioration of fixing due to miniaturization of a diameter of a toner. As a proposal other than a toner bulk, there is proposed a toner wherein an inorganic compound with the surface treated with low-molecular polyethylene is added to the surface of the toner (see JP-A No. 5-165250). However, this proposal does not refer to the fixing property. Moreover, the surface of the treated inorganic compound is not uniformly covered with polyethylene, and a large amount of polyethylene does not cover the surface of an inorganic compound. For this reason, the contamination of the surface of a photosensitive member with polyethylene itself is observed in long term and improvement in the fixing property is insufficient.
A dry developer can be roughly classified into a one-component developer using a toner in which a colorant is dispersed into a binder resin, and a two-component developer in which a carrier is mixed into the toner. In any case, upon copying, an electrostatic latent image formed on a photosensitive member is developed by these developers, and a toner image on the surface of a photosensitive member is transferred and, thereafter, a toner remaining on the surface of a photosensitive member is cleaned. Therefore, it is required that a dry developer satisfies various conditions in a copying step, in particular, in a developing step and a cleaning step. A toner is supplied to developing not as a flocculate but as individual particles. It is thus required that a toner has the sufficient flowability and, at the same time, this flowability or electric properties do not change depending on time or the environment (temperature, humidity). In addition, in a two-component developer, it is required that adhesion of a toner to the carrier surface, so-called toner filming phenomenon does not occur.
Further, upon cleaning, such the cleaning property is required that a remaining toner is easily dropped off from the surface of a photosensitive member and, when used with a cleaning member such as a blade, a web and the like, a photosensitive member is not damaged. In order to satisfy these various requirements, in a dry developer, there are variously proposed a one-component developer or a two-component developer in which an inorganic fine powder such as silica and the like, an organic fine powder such as fatty acid, or a metal salt or a derivative thereof, or a fluorine series resin fine powder is externally added to a toner, and the flowability, the durability or the cleaning property is tried to be improved.
However, among previously proposed additives, although an inorganic compound such as silica, titania, alumina and the like remarkably improves the flowability, there is a problem that a recess or a flaw is easily produced on a surface layer of a photosensitive member by a hard inorganic compound fine powder, and toner adhesion is easily caused at a damaged part. In addition, recently, although utilization of a recycled paper for the purpose of source saving has been increased, there is generally a problem that a recycled paper produces much paper powder, a paper powder enters between a photosensitive member and a blade, and induces deteriorated cleaning such as black streak.
In order to solve these problems, a fatty acid metal salt as an additive is externally added to a toner, or a wax is externally added (for example, see JP-A Nos. 60-198556, 61-231562 and 61-231563). In these cases, a particle diameter of an additive is large as 3 to 20 μm in all cases and, in order to effectively manifest the effects, it becomes necessary to add at a considerable amount. In addition, although effective initially, there arises a problem that formation of a membrane as a lubricant is not uniform due to unique filming of an additive (lubricant), and white spot, image fading and the like occur in an image.
In addition, as an additive, there are provided a titanium oxide particle treated with a fatty acid metal salt, a titanium oxide fine particle with the surface treated in hydrolyzing a fatty acid compound in an aqueous system, an inorganic compound with the surface treated with a fatty acid metal salt and fine particle titanium oxide obtained by hydrophobilization by surface treatment with fatty acid aluminium (for example, see JP-A Nos. 4-452, 5-66607, 5-165250, 10-161342). By using a fatty acid metal salt in the surface treatment like this, the aforementioned problems derived from a magnitude of a particle diameter of a fatty acid metal salt itself are avoided to some extent. However, no case refers to the uniformity of the surface treatment, and prevention of a flaw on the surface of a photosensitive member is insufficient.
On the other hand, a method of preventing toner filming by externally adding a hydrophobic hard fine powder to a toner, and cutting a photosensitive member by the polishing effect of a hard fine powder is proposed, for example, in JP-A No. 2-89064. However, although this method is effective in suppression of filming, there is a defect that the surface of a photosensitive member is abraded, and a photosensitive member's life is remarkably reduced. At the same time, there is also a defect that a cleaning blade is abraded with a hard fine powder, and a life of a blade is remarkably reduced.